Surface coatings are used on consumer devices to protect the surface and enhance the aesthetics and texture of the device. One example of such a coating is anodizing a metal surface. Anodizing a metal surface converts a portion of the metal surface into an anodic oxide, thereby creating an anodic oxide layer. The anodic oxide layer may be harder than the underlying metal substrate.
While a coating may be harder than the underlying substrate, a relatively stiff coating is susceptible to becoming detached from the substrate when the article is subjected to mechanical stresses. Thick, stiff coatings, on relatively compliant substrates are particularly susceptible to delamination, and since properties such as stiffness and thermal expansion are often strongly correlated, this scenario can be exacerbated by thermally induced strain.
Various mechanical tests exist for evaluating the interfacial strength and interfacial adhesion of the coatings. These include pull-off tests, thermal cycling and thermal shock, and techniques such as four-point bend delamination for propagating delamination under steady state and measuring interfacial adhesion energies. One common (though more qualitative) test for the adhesive strength of a coating of consumer products is the “rock tumble” test. This test is performed by tumbling the article having the coating for an extended time with items the article will typically encounter during its expected lifetime. However, these tests are random in nature, and must be repeated multiple times for each coating to be tested to extrapolate a statistically significant result. In addition, these tests may have inherent limitations, such as the strength of adhesives used for bonding in pull-off tests, or require very specific sample geometries, such as is the case in bend delamination tests. For these reasons, incremental improvements in layered structure strength and adhesion are difficult to evaluate.